1. Field of the Invention
The present invention relates to a novel composition of high frequency dielectric ceramics. More particularly, the present invention relates to compositions of high frequency dielectric ceramics having high Q values, high dielectric constants, and low temperature coefficients of the resonant frequency.
2. Description of the Prior Art
Recently, various communication systems using microwave frequencies with a range of 300 MHz to 300 GHz have been developed. These include mobile radio communication systems such as wireless telephone sets, carphones, satellite broadcasting systems, satellite communication systems and the like. Such systems require high frequency dielectric ceramics applicable to resonators, band-pass (or rejection) filters, microwave integrated circuits (MICs) and the like, and demand for such components has greatly increased. A high frequency dielectric ceramics to be applied to such communication systems should have the following characteristics: (1) a high dielectric constant, which is required for miniaturization of parts made of dielectric ceramics because the wavelength of microwaves within dielectric ceramics is inversely proportional to the square root of the dielectric constant; (2) a high Q value corresponding to a reciprocal of the dielectric constant, which is required for high performance because dielectric loss is directly proportional to frequency; and (3) a low temperature coefficient of the resonant frequency of a dielectric resonator. See, W. Wersing, "Electronic Ceramics", B.C.H Steele ed., p. 67, Elsevier Sci. Pub. Co., New York (1991). In addition, high frequency dielectric ceramics to be applied to such communication systems should have a small change in properties with time, high thermal conductivity, and good mechanical strength.
Examples of dielectric ceramics which have been heretofore developed include a Ba(M.sup.+2.sub.1/3 M.sup.+5.sub.2/3) O.sub.3 system wherein M.sup.+2 is Mg or Zn, and M.sup.+5 is Ta or Nb, a Ba.sub.2 Ti.sub.9 O.sub.20 system, and a (Zr,Sn)TiO.sub.4 system. These types of dielectric ceramics have low dielectric losses, while they have dielectric constants less than about 40. Other examples include a BaO-Sm.sub.2 O.sub.3 -TiO.sub.2 system, a (Ba,Pb)O-Nd.sub.2 O.sub.3 -TiO.sub.2 system, and a (Pb,Ca)ZrO.sub.3 system. These types of dielectric ceramics have dielectric constants of not less than about 80, while they have relatively high dielectric losses, for example Qxf.sub.0 (GHz)&lt;about 10,000. See, W. Wersing supra and J. Kato, JEE, Sep., pp. 114-118 (1991).
Generally, dielectric materials having high dielectric constants exhibit an increased dielectric loss and a temperature coefficient of the resonant frequency due to the dipoles and the defects associated therewith. However, for most applications of high frequency dielectric ceramics, they must have stable temperature coefficients of the resonant frequency.
SrTiO.sub.3 has a very high dielectric constant of about 255 at 2 GHz, but has a very high temperature coefficient of the resonant frequency of about +1,670 ppm/.degree. C. La(Zn.sub.(1-x)/2 Mg.sub.x/2 Ti.sub.1/2)O.sub.3 has a very high Qxf.sub.0 (GHz) of about 80,000 to about 91,000 and a low temperature coefficient of the resonant frequency ranging from about -55 to about -70 ppm/.degree. C., but has a low dielectric constant of about 28 to about 31.